MARYLAND GEOLOGICAL SURVEY 619 



viduals, feebly depressed in front of the obscure carina which extends 

 from the umbones toward the posterior ventral margin, the depression 

 being reflected in the slight concavity of the base ; posterior end strongly 

 and symmetrically arcuate; dorsal margin approximately horizontal; 

 external surface smooth excepting for the incremental sculpture which is 

 rather conspicuous, particularly in the posterior portion of the shell; 

 characters of interior not known. 



The species frequently occurs in clusters, the individuals being attached 

 at the posterior extremity. The degree of medial constriction is not 

 constant. 



L. ripleyana Gabb is relatively more elongated transversely than L. 

 affinis Gabb, a co-existent species over much of the area of its occurrence, 

 and is much less inflated. 



Occurrence. MATAWAN FORMATION. Opposite Post 239, Post 236, 

 Camp Fox, Chesapeake and Delaware Canal, Delaware. MONMOUTH FOR- 

 MATION. Bohemia Mills, Cecil County; Brightseat, Brooks estate near 

 Seat Pleasant, Friendly, Prince George's County, Maryland. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. Matawan Formation. Merchantville clay marl, 

 Wenonah sand, New Jersey. Monmoutli Formation. Navesink marl, New 

 Jersey. Ripley Formation. Exogyra costata zone, Union County, Mis- 

 sissippi. 



LlTHOPHAGA CONCHAFODENTIS n. Sp. 



Plate XXXVI, Figs. 7-9 



Description. Shell nacreous in texture, moderately large for the genus, 

 subcylindrical to rectangular in outline, exceedingly thin and fragile; 

 umbones nearly terminal, small, full but angular, flattened upon their 

 summits, acute, prosogyrate ; posterior area cut off by a carina which per- 

 sists from the umbones to the posterior basal margin, acute near the 

 umbones, but evanescing toward the base; anterior end very short and 

 obscurely truncate ; posterior end much produced, strongly rounded at its 

 extremity; the dorsal and ventral margins rudely parallel, the dorsal 



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